It is known to apply a mechanical shock to a pile, detect the resulting vibrations of the pile, and analyse them to provide data about the pile. Such methods are known variously as `acoustic`, `sonic`, or `transient dynamic` pile survey. Essentially, the method measures the pile-head's vertical motions in response to a mechanical shock. The shock may be generated by a manually-delivered blow downwards on the pile-head, sometimes via a `load cell` which measures the force of the initial shock, and the subsequent vertical motions of the pile-head, or it may be that the concrete is struck directly and the resulting motions are registered by a velocity or acceleration transducer firmly attached to the concrete nearby the shock position.
The shock causes the concrete to be momentarily compressed, and vertical motions of waveform approximating a decaying sinusoid follow the blow, as measured at the pile-head. In descending the column of the pile, reflection of proportions of the shock wave occur wherever the column varies in cross-sectional area, or in density. On returning to the pile-head such reflections `interfere` with the decaying sinusoid motions there, and interpretation of this resultant complex waveform enables predictions as to the existence of defects in the pile column, and confirmation or otherwise that it is of the correct length. The technique is now well-established in the Construction Industry.
There are three essential conditions for the acquisition of the pile shock vibration data:
1. the vibration transducer must be held in firm contact with the concrete of the pile-head
2. a shock (or `impulse`) must occur
3. a record of the vertical vibration motions of the transducer (and thus the pile-head) must be made
In addition, a means should exist for recording the identity (its site number) of the pile under test.
In many site circumstances, access to the piles may not be straightforward. They may be relatively distant from a position to which a vehicle can be conveniently brought, or the pile-heads may be covered in mud, water or other site debris, and site plant and other obstacles may be inimical to the presence of the recording apparatus and the normal preparations necessary for the tests to proceed.
Existing methods are slow in execution, and may be difficult or impossible to carry out in the face of such usual construction site obstacles as mud, flooding, deep trenching, shuttering, site traffic and so on.